1. Field of the Invention
The present invention relates to an alloy type thermal fuse of an operating temperature of about 130 to 170° C., and a wire member for such a thermal fuse element.
2. Description of the Prior Art
An alloy type thermal fuse is widely used as a thermo-protector for an electrical appliance or a circuit element, for example, a semiconductor device, a capacitor, or a resistor.
Such an alloy type thermal fuse has a configuration in which an alloy of a predetermined melting point is used as a fuse element, the fuse element is bonded between a pair of lead conductors, a flux is applied to the fuse element, and the flux-applied fuse element is sealed by an insulator.
The alloy type thermal fuse has the following operation mechanism.
The alloy type thermal fuse is disposed so as to thermally contact an electrical appliance or a circuit element which is to be protected. When the electrical appliance or the circuit element is caused to generate heat by any abnormality, the fuse element alloy of the thermal fuse is melted by the generated heat, and the molten alloy is divided and spheroidized because of the wettability with respect to the lead conductors or electrodes under the coexistence with the activated flux that has already melted. The power supply is finally interrupted as a result of advancement of the division and spheroidization. The temperature of the appliance is lowered by the power supply interruption, and the divided molten alloys are solidified, whereby the non-return cut-off operation is completed. Therefore, the division temperature of the fuse element alloy is set to be substantially equal to the allowable temperature of an electrical appliance or the like.
It is known that an alloy type thermal fuse having the operation mechanism is requested to have the overload characteristic and the dielectric breakdown characteristic.
The overload characteristic means external stability in which, even when a thermal fused operates in increased ambient temperature under the state where a current and a voltage of a given degree are applied to the thermal fuse, the fuse is not damaged or does not generate an arc, a flame, or the like, thereby preventing a dangerous condition from occurring. The dielectric breakdown characteristic means insulation stability in which, even at a given high voltage, a thermal fuse that operates does not cause dielectric breakdown and the insulation can be maintained.
A method of evaluating the overload characteristic and the dielectric breakdown characteristic is specified in IEC (International Electrotechnical Commission) Standard 60691 which is a typical standard, as follows. When, while a rated voltage×1.1 and a rated current×1.1 are applied to a thermal fuse, the temperature is raised at a rate of 2±1 K/min. to cause the thermal fuse to operate, the fuse does not generate an arc, a flame, or the like, thereby preventing a dangerous condition from occurring. After the thermal fuse operates, even when a voltage of (the rated voltage×2+1,000 V) is applied for 1 min. between a metal foil wrapped around the body of the fuse and lead conductors, and, even when a voltage of the rated voltage×2 is applied for 1 min. between the lead conductors, discharge or dielectric breakdown does not occur.
Because of recent increased awareness of environment conservation, the trend to prohibit the use of materials harmful to a living body is growing as a further requirement on an alloy type thermal fuse. Also a fuse element for such a thermal fuse is requested not to contain a harmful material.
Moreover, an alloy type thermal fuse is requested to increase the capacity and reduce the size in order to comply with the enhanced speed, the increased number of functions, and the miniaturization of an electronic device.
As an alloy type thermal fuse in which the operating temperature belongs to the range of about 130 to 170° C., conventionally, known are a fuse of a ternary In—Pb—Sn alloy (for example, Japanese Patent Application Laying-Open Nos. 11-73869, 59-8231, and 3-236130), that of a ternary Sn—Pb—Cd alloy, that of a binary Bi—Sn alloy (for example, Japanese Patent Application Laying-Open No. 2002-25405), and the like.
However, the former two fuses contain metals harmful to the ecological system such as lead and cadmium, and hence are inadequate under the recent strict environmental regulations.
By contrast, a fuse of a binary Bi—Sn alloy has a narrow solid-liquid coexisting region, and contains a large amount of Bi, so that the surface tension of the fuse tends to be high. Therefore, such a fuse has the following overload and dielectric breakdown characteristics. Because of the narrow solid-liquid coexisting region, the alloy during energization and temperature rise may be suddenly changed from the solid phase to the liquid phase, thereby causing an arc to be easily generated immediately after the operation. When an arc is generated, a local and sudden temperature rise occurs. As a result, the flux is vaporized to raise the internal pressure, or the flux is charred. In addition to the above, also the high surface tension causes the molten alloy or the charred flux to be intensely scattered as a result of an energizing operation. Therefore, physical destruction such as generation of cracks due to a local and sudden temperature rise, a rise of the internal pressure, or reconduction between charred flux portions easily occurs during an operation. Moreover, the insulation distance is shortened by the scattered alloy or the charred flux, so that dielectric breakdown is easily caused by reconduction when a voltage is applied after an operation. Furthermore, the alloy structure is slightly weak and has poor ductility. Therefore, the alloy is hardly thinned to, for example, 200 μmφ, and hence is not meet the requirement of a thinned thermal fuse.
A ternary Sn—In—Bi alloy contains no metal harmful to the ecological system, and hence satisfies environment conservation.
Conventionally, various alloy type thermal fuses in which a ternary Sn—In—Bi alloy is used as a fuse element have been proposed.
For example, known are a fuse which has an alloy composition of 42 to 53% In, 40 to 46% Sn, and 7 to 12% Bi, and in which the operating temperature is 95 to 105° C. (Japanese Patent Application Laying-Open No. 2001-266724), that which has an alloy composition of 55 to 72.5% In, 2.5 to 10% Sn, and 25 to 35% Bi, and in which the operating temperature is 65 to 75° C. (Japanese Patent Application Laying-Open No. 2001-291459), that which has an alloy composition of 51 to 53% In, 42 to 44% Sn, and 4 to 6% Bi, and in which the operating temperature is 107 to 113° C. (Japanese Patent Application Laying-Open No. 59-8229), that which has an alloy composition of 1 to 15% Sn, 20 to 33% Bi, and the balance In, and in which the operating temperature is 75 to 100° C. (Japanese Patent Application Laying-Open No. 2001-325867), and that which has an alloy composition of 0.3 to 1.5% Sn, 51 to 54% In, and the balance Bi, and in which the operating temperature is 86 to 89° C. (Japanese Patent Application Laying-Open No. 6-325670).
However, all the proposed fuses have an operating temperature which is lower than 130° C., and cannot satisfy the requirement that the operating temperature is 130 to 170° C.
Also a fuse which has an alloy composition of 0.5 to 10% In, 33 to 43% Sn, and 47 to 66.5% Bi, and in which the operating temperature is 125 to 135° C. is known (Japanese Patent Application Laying-Open No. 2001-266723). With respect to the range of higher than 135° C. and not higher than 170° C., however, the fuse cannot satisfy the requirement that the operating temperature is 130 to 170° C.